The present invention relates to a signal processing circuit for a composite color television signal such as NTSC system, and in particular concerns a YC-signal separation circuit and an enhancement circuit of the motion adaptive type.
Studies and development of digital televisions in an effort to realize a high image quality and high performance by making use of semiconductor memories and digital signal processing techniques are in full flourish. In the signal processing in such digital television technology, the YC-signal separation (i.e. separation of luminance and color difference signals Y and C) plays an important role. An example of the hitherto known YC-signal separation processings is disclosed in Japanese Patent Application Laid-Open No. 115995/1983 (JP-A-58-115995). According to this known technique, motion of a picture is detected by writing and reading the television signal in and from a frame memory to thereby determine whether the picture in concern is a still picture or a motion picture. In a case of the still picture, the YC-signal separation is performed by making use of an inter-frame correlation, while for a motion picture the YC-signal separation is carried out by making use of an inter-line correlation (vertical correlation). However, in the YC-signal separation circuit disclosed in the above mentioned publication (JP-A-No. 58-115995), no consideration is paid to the cross-talk taking place between the Y-signal and the C-signal upon the signal separation being carried out for a motion picture having no vertical correlation.
It is further noted that in the hitherto known television apparatus, image quality undergoes degradation resulting from disturbances such as cross-color phenomenon, dot interference and interlacing interference. In recent years, numerous and various approaches have been made for developing digital televisions in which adaptive YC-signal separation, adaptive scanning line interpolation and enhancement are performed by making use of a frame memory and advanced digital signal processing with the aim for realizing reproduction of a high quality image by eliminating the aforementioned disturbance factors which bring about degradation in the image quality. The enhancement processing is performed for realizing a high image resolution. However, the hitherto known enhancement techniques suffer problems that realization of high image resolution is accompanied with occurrence interlacing interference and dot interference of considerable magnitude.
The problems of the hitherto known enhancement technique will be discussed below in some detail by taking as an example an NTSC color television signal. First, description is directed to a horizontal enhancement circuit. The YC-signal separation of the motion adaptive type is accomplished by changing over the filtering operation with respect to the directions corresponding to the time axis, vertical axis and the horizontal axis, respectively. More specifically, in the case of a still picture, the YC-signal separation can be realized by using a time-axis filter without any accompanying cross-talk. In the case of a motion picture having a vertical correlation, the YC-signal separation can be effectuated by using a vertical-axis filter without any accompanying cross-talk. However, in the case of a motion picture exhibiting no vertical correlation, the YC-signal separation cannot be carried out without being accompanied with cross-talk between different filters. The hitherto known enhancement circuit is so designed that the gain is increased in the high frequency range of the luminance signal Y (e.g. in the vicinity of 3.58 MHz) independent of whether a motion or a still picture is to be processed or whether or not vertical correlation is present. Consequently, the high resolution is necessarily accompanied with significant dot interference. A vertical enhancement circuit will be considered. According to the adaptive scanning line interpolation, the processings performed by the time-axis filter and the vertical-axis filter are changed over to remove the fly-back signal in the interlaced scanning which lies in a frequency space (30 Hz, 525 lines). FIG. 29 of the accompanying drawings illustrates an interpolation processing for a still picture, FIG. 30 illustrates an interpolation processing for a motion picture having a vertical correlation, and FIG. 31 illustrates an interpolation processing for a motion picture having no vertical correlation. In FIGS. 29 to 31, a reference numeral 111 designates a base band signal, 112 designates a fly-back signal in the interlaced scanning, 113 designates a frequency band eliminated through the filtering processing, and 114 designates a fly-back signal admixed with the base band. In the hitherto known vertical enhancement circuit, the gain is increased in a vertical high frequency range of the luminance signal (e.g. in the vicinity of 262.5 television lines). Consequently, realization of high resolution is accompanied with significant interlacing interference due to the fly-back signal 114.
An enhancement circuit of this type is disclosed in Japanese Patent Application Laid-Open No. 19365/1985 (JP-A-No. 60-19365). Furthermore, a known extended definition TV system is disclosed in the publication "Extended Definition TV Fully Compatible with Existing Standards" TAKAHIRO FUKINUKI ET AL, IEEE Transactions on Communications, Vol. COM-32, No. 8, August 1984. Furthermore, a known motion-compensated prediction system is disclosed in the publication "A Motion-Compensated Interframe Coding Scheme for Television Pictures" YUICHI NINOMIYA ET AL, IEEE Transactions on Communications, Vol. COM-30, No. 1, January 1982.